There is a trend today in the automobile industry to operate the various vehicle accessories, such as the power steering pump, oil and air pumps, air conditioning and alternator, by a single endless belt driven by a pulley connected to the engine crankshaft. This system is referred to as a "serpentine" drive belt system. To ensure optimum operating efficiency for these various accessories, it is necessary that the driven belt be maintained at a predetermined tension to assure efficient performance of the accessories as well as satisfactory service life for the belt. Due to the relatively greater length for the single drive belt which replaces the heretofore plurality of smaller belts, there is a greater tendency for the belt to stretch which will affect the operating characteristics of the driven accessories. Therefore, it is desirable that a belt tensioning device be used for these endless belts to provide reliable service over an extended period of time and to maintain a constant amount of tension on the belt regardless of the amount of belt stretch.
Numerous devices have been proposed and used to accomplish this purpose. One type of tensioner uses a bushing formed of an elastomeric material which is placed in compression by some mechanical means for continuously exerting a tensioning force on the belt. Examples of these constructions are shown in U.S. Pat. Nos. 3,975,965 and 4,144,772. These tensioning constructions have the disadvantage that the bushing must serve as the spring, provide the required damping and also maintain the alignment of the arm and all of these functions are therefore compromised. The spring rate compromise results in belt tensioner variation and the damping compromise results in a lack of motion control. Also, the bushing softness allows the arm to deflect resulting in less alignment control of the arm and pulley.
Numerous other types of belt tensioning devices use coil springs which are either in compression or tension, for applying and maintaining the tensioning force on a belt-engaging idler pully or chain-engaging sprocket. Some examples of these types of constructions are shown in U.S. Pat. Nos. 2,703,019; 2,893,255; 3,413,866; 3,483,763; 3,631,734; 3,768,324; 3,812,733; 3,924,483; 3,965,768 and 4,108,013. Some of these various coil spring-actuated devices use the biasing force of a spring in combination with hydraulic-actuated members for regulating the amount of tensioning force applied to the belt, depending on whether the engine is running or shut off. Examples of these combination spring and hydraulic belt tensioners are shown in U.S. Pat. Nos. 2,051,488; 3,142,193; and 3,077,272.
Other known belt tensioner constructions, such as shown in U.S. Pat. No. 3,924,483, use a torsional spring for pivotally moving one of the vehicle accessories to achieve the desired tensioning force. Other constructions, such as shown in U.S. Pat. Nos. 3,136,170; 3,483,763; 3,834,246; and 4,285,676, use a torsional coil spring for pivotally moving a lever and idler pulley into belt tensioning engagement which provides a relatively simple, economical and compact unit. U.S. Pat. No. 4,473,362 shows still another belt tensioner which uses a torsional coil spring to provide a variable damping force by applying the radial forces exerted by the volutes of the spring against an internal elastomeric bushing.
It is desirable that a belt tensioner be provided with some type of damping means to prevent excessive oscillation from occurring in the spring tensioning member, and which will absorb sudden shocks to prevent a whipping action from occurring in the tensioner and drive belt. This damping action is especially critical when a coil spring is used for applying the belt tensioning force since coil springs are highly susceptible to developing natural oscillating frequencies when the counter force, which is exerted thereon by the belt, fluctuates during acceleration action. Such fluctuations effect the efficiency of the tensioning force applied to the belt by the coil spring and reduces belt life.
Various damping devices have been used with belt tensioners to eliminate or reduce this problem of coil spring oscillation. One type of construction uses a hydraulic fluid as the damping means, such as shown in U.S. Pat. Nos. 2,893,255; 3,964,311 and 3,986,407. U.S. Pat. No. 3,710,634 shows a belt tensioner which uses an eccentrically mounted mechanical pinion and rack arrangement which is spring biased by a leaf spring for absorbing an excessive amount of shock as opposed to providing a damping action for spring-biased belt tensioning plunger.
It also is high desirable when developing a belt tensioner intended primarily for use on an automobile to devise a construction which can be produced as inexpensively as possible without sacrificing durability and efficiency since a savings of only part of a dollar would amount to a sufficient overall savings when considering the millions of vehicles that are produced by the various vehicle manufacturers on which such belt tensioners will be mounted.
Many of these problems have been overcome by the belt tensioner which is the subject of patent application Ser. No. 598,043, filed Apr. 9, 1984, by a coinventor of the present application. In the belt tensioner of this application, a damping ring is biased into frictional engagement with the lever arm on which an idler pulley is mounted by the same torsional coil spring which provides the tensioning force on the lever arm. Such an arrangement insures that the amount of damping is proportional to the torque of the spring. Although the device of this pending application solves many of the problems, it is desirable to form the damping ring of a material suitable to achieve the most efficient frictional engagement between the pivotal movable lever arm and a stationary part of the bracket while in addition to providing a damping band of a material able to withstand the various forces exerted thereon by the biasing force of the torsional spring. Furthermore, it is desirable that the amount of damping force applied by the damping band from the torsional coil spring be changable without materially changing the component parts of the tensioner.
Therefore, the need has existed for such a belt tensioner having an improved damping arrangement which uses a damping band formed out of the most satisfactory and efficient friction material; in which the damping band is moved into damping engagement by the torsional spring which provides the belt tensioning force; in which the damping band is of such a material which is able to better withstand the forces exerted thereon by the coil spring; and in which the amount of tension exerted by the torsional spring on the damping band can be varied without materially affecting the biasing force exerted by the spring on the idler pulley lever arm.